A brief discussion on the safety design of smart door lock clutches

When it comes to the difference between smart door locks and traditional door locks, everyone can easily come up with examples, such as fingerprint, password, Bluetooth, NFC, and some even have 4G/5G modules. However, many people ignore that smart door locks also have a special structure in terms of mechanics - the clutch.

Taking the handle door lock as an example, the so-called clutch refers to the power transmission switch between the external handle and the bolt. Generally speaking, when the clutch is in the disengaged state, twisting the external handle can only rotate in vain and cannot open the door. When the clutch is in the engaged state, it can drive the bolt to open the door.

To design a clutch, the thoughts of engineers are diverse, and it is inevitable that some mistakes are made intentionally or unintentionally, which may expose users to danger. Therefore, the author has sorted out and analyzed the common design schemes of door lock clutches on the market, hoping to attract the attention of security researchers and ordinary consumers.

I.Classification according to whether there is a clutch

* With clutch:

Most smart door locks have a clutch, such as the Xiaomi Aijia smart door lock series, which all belong to this type. After receiving an unlocking command, manual twisting of the external handle or pulling the door handle is required to open the door.

* No clutch:

A few smart door locks use an automatic lock body, that is, the lock body is built-in with a motor. After receiving an unlocking command, the motor drives the bolt to open the door without external force, which is slightly better in terms of experience, with high anti-theft safety. However, due to the use of an electronic lock body, reliability decreases, and it may not be possible to open the door electronically in scenarios such as fires and dampness, so it is usually necessary to reserve a mechanical door opening channel as a backup. On the other hand, the motor consumes a relatively large amount of electricity.

II. Classification according to the mechanism of clutch action

* Connecting-type clutch:

That is, what is usually called a clutch. In the separated state, it is impossible to open the door by twisting the handle, and only when it is in the engaged state can it drive the bolt to open the door. Unless otherwise specified, all clutches mentioned in the following text belong to this type.

* Blocking-type clutch:

Strictly speaking, it does not conform to the definition of a clutch (a mechanism for connecting and disconnecting an engine and the transmission system), which is generally a metal pin. In the normal state without receiving an unlocking command, the pin is inserted into the steel sleeve to prevent its rotation, which is intuitively described as the handle cannot be twisted.

It is evident that the safety of this clutch depends on the physical strength of the pin, however, there are many low-strength metal pins on the market. Attackers can simply twist the door handle to open the door, and this design has a hundred disadvantages and no benefits, and should be eliminated.

Third.Classification according to the installation position of the clutch

* External clutch:

The clutch is located on the front panel, and the structure is very simple, as shown in the following case. When no unlocking command is received, the bolt pin is not inserted into the clutch axis, so twisting the handle will not cause the square steel to rotate.

Generally speaking, dummy core door locks will adopt this design, which can reduce the production cost of enterprises and the maintenance cost for users is also very small.

However, it is obvious that this design is similar to placing the password judgment logic in the client JavaScript of a web login system. The attacker only needs to bypass the clutch to open the door, either by prying open the front panel to manually connect the clutch or by prying a gap between the front panel and the door panel, using a tool to clamp the square steel and twist it, which is not a problem.

* Centralized clutch:

The clutch is located inside the lock body. Unlike other schemes, it is no longer a square steel rod passing through the front and rear panels, but is divided into an external square steel and an internal square steel, as shown in the following case. When no unlocking command is received, twisting the external handle can only drive the clutch block to rotate and cannot open the door. After receiving the unlocking command, the clutch pin is inserted into the clutch hole, the external square steel drives the clutch block, and the clutch block can drive the pin block to rotate, further controlling the unlocking mechanism to open the door.

It can be seen that even if the front panel is removed, the attacker cannot open the door, which has high anti-theft safety. However, similarly, due to the power lines and signal lines connected to the lock body, the reliability decreases, which may lead to the inability to open the door electronically in scenarios such as fires and dampness.

* Internal clutch:

The clutch is located on the rear panel, as shown in the following case. The square steel rod passes through the lock body, but it cannot directly control the bolt. It must be controlled by the internal handle turning part. When no unlocking command is received, the square steel sleeve and the internal handle turning part are in a separated state. Therefore, twisting the outer handle of the door will only drive the square steel and the square steel sleeve to rotate, and cannot drive the internal handle turning part to further control the bolt to open the door. After receiving the unlocking command, the power device pushes the pin to the side round hole on the square steel sleeve, connecting the square steel sleeve with the internal handle turning part. Twisting the outer handle of the door can then drive the entire transmission system to open the door.

As can be seen, this design places the clutch on the rear panel, which has high anti-theft safety. At the same time, it does not require an electronic lock body, and it will not compromise the stability and reliability.

Note:The clutch is just one of the components of the smart lock, and the safety of the clutch cannot directly affect the overall safety of the smart lock. This article cannot be the sole basis for choosing a lock.

References

[1]. Liu Renxiang, Feng Ye, Liang Longqu, CN208294262U An Electronic Lock Composite Clutch

[2]. Cai Yongliang, CN209568792U A Lock Body Clutch Device

[3]. Chen Zhenghua, Ni Huizhen, Liu Lijun, Lu Rong, Meng Tao, CN204663159U A Top Plate Return Clutch Component